john hawks weblog

paleoanthropology, genetics and evolution

North Africa

  • Behavior of the first North African humans

    Thu, 2013-03-07 23:49 -- John Hawks

    Mohamed Sahnouni and colleagues describe the archaeology of El-Kherba, Algeria. [1]. This locality is a paleontological exposure associated with the nearby Ain Hanech site, and Sahnouni and colleagues have excavated an Oldowan archaeological assemblage with large mammals such as hippos, rhinos and horses.

    Dated to 1.78 Ma, the El-Kherba cut marks and usewear traces represent the earliest North African evidence showing a clear causal link between Oldowan stone technology and processing of large animal carcasses for meat, broadening the geographic range of Plio-Pleistocene hominin subsistence activities to include the Mediterranean fringe. As was shown in the East African Plio-Pleistocene archaeofaunas, early hominins were foraging for large mammals in northern Africa by circa 1.8 Ma. The evidence from the modified bones at these sites indicates that early hominins were involved in evisceration, disarticulating and removing meat, and breaking bones of large mammals to extract marrow.

    It's a great site because it is the first to document human activity in North Africa. Australopithecines were present in Chad by 3.4 million years ago, and given their mobility and range it seems likely they would have been present to the north of the Sahara also. But none have ever yet been found. As it stands, humans were at Dmanisi by 1.78 million years ago and also in Java by that time. The extent of human migration outside of Africa makes it clear that the Mediterranean coast of Africa itself should have been well within their range.

    And yet, stone tools are known from Ethiopia from 2.6 million years ago, and nearly as old in Kenya. Did the earliest stone toolmakers range beyond the Rift Valley? So far there's no equivalently early evidence of tool manufacture in South Africa. And in North Africa, the earliest tool assemblage is at El-Kherba.

    It would sure be useful to uncover evidence of A. boisei or related robust australopithecines in the Ain Hanech area. In East and South Africa, early Homo lived alongside late robust australopithecines, sharing the same landscape. No robust australopithecine has ever been found outside East or South Africa, while Homo erectus spread across the Old World tropics and into the temperate zone. What kept robust australopithecines, otherwise seemingly adaptable, out of Eurasia? If they truly never lived near the Mediterranean coast, we would probably conclude that they weren't as tolerant of different habitats as we might have expected.

    The cutmark evidence described in the paper is fairly clear and comparable to that known from East Africa well before this date. The cutmarks on animal bones, including hippopotamus, along with a "meat polish" on some of the stone flakes, indicate that ancient humans had access to animal carcasses very shortly after the animals' death and were using stone flakes to process them. Again, basically like Oldowan evidence that has long been known from Olduvai Gorge and other sites. I would like to see a better comparison of where this assemblage fits compared to both large and small archaeological assemblages from Olduvai.

    The question of whether and to what extent early humans hunted large mammals involves a long debate that wouldn't fit well in this paper. Still, the evidence here adds to that literature. The ancient people who left these remains were relying upon large mammal acquisition within a broader hunted diet including smaller prey species. Together with sites from across Africa and Eurasia, this one shows that early humans maintained this diet pattern across a range of ecologies and geographies.


    References

    Synopsis: 
    Archaeological report from El-Kherba, Algeria, with implications for human occupation range
  • The North African Neandertal descendants

    Thu, 2012-10-18 16:25 -- John Hawks

    A new paper by Federico Sánchez-Quinto and colleagues reports on comparisons of North African population samples with the Neandertal DNA project data [1]. The paper shows that North African populations also carry a substantial trace of Neandertal ancestry, like living populations outside of Africa, much more than populations of sub-Saharan Africa.

    One of the main findings derived from the analysis of the Neandertal genome was the evidence for admixture between Neandertals and non-African modern humans. An alternative scenario is that the ancestral population of non-Africans was closer to Neandertals than to Africans because of ancient population substructure. Thus, the study of North African populations is crucial for testing both hypotheses. We analyzed a total of 780,000 SNPs in 125 individuals representing seven different North African locations and searched for their ancestral/derived state in comparison to different human populations and Neandertals. We found that North African populations have a significant excess of derived alleles shared with Neandertals, when compared to sub-Saharan Africans. This excess is similar to that found in non-African humans, a fact that can be interpreted as a sign of Neandertal admixture. Furthermore, the Neandertal's genetic signal is higher in populations with a local, pre-Neolithic North African ancestry. Therefore, the detected ancient admixture is not due to recent Near Eastern or European migrations. Sub-Saharan populations are the only ones not affected by the admixture event with Neandertals.

    The interesting aspect of the paper is that the authors attempted to separate the ancestry of North African samples into a pre-Neolithic indigenous African component, and a residual component that represents more recent gene flow into North Africa, from all sources. The historic movement into North Africa has been fairly cosmopolitan, involving sub-Saharan Africans, Arabs, Medieval Europeans, Romans, Carthaginians and many other peoples. Sánchez-Quinto and colleagues used the ADMIXTURE program to try to sort out a pre-Neolithic indigenous component and analyze that specifically for Neandertal similarity.

    Unsurprisingly, the fraction of estimated sub-Saharan African ancestry in each population sample was inversely correlated with the estimated Neandertal ancestry. That is, the more a population looks like sub-Saharan Africans, the less Neandertal it has.

    Here's what's surprising: When they sorted out parts of the genome in Tunisians that ADMIXTURE determines to be most likely from pre-Neolithic North Africans, they found these parts of the genome had more Neandertal ancestry than typical of the CEU sample of northern European ancestry. Is it possible that ancient North Africans had more Neandertal similarity than today's Europeans?

    Sánchez-Quinto and colleagues suggest that the Neandertal ancestry in this population came in Upper Paleolithic times from the Near East. That is possible, or some of the Neandertal similarity may reflect ancient African population structure. Really I think we will have to do a finer analysis of chromosome blocks to examine the subset of shared Neandertal derived alleles that reflect introgression versus incomplete sorting from the ancestral African population. It will be very interesting to examine more closely the mixture of population history within Egypt, through which most Near Eastern pre-Neolithic population movement must have come.

    The authors note that the distribution of Neandertal similarity outside Africa increases with distance from Africa.

    A previous study [26] observed that the similarity to Neandertals increases with distance from Africa and suggested this could be explained by SNP ascertainment bias plus a strong genetic drift in East Asian populations. Nonetheless more complex, population-biased, ascertainment schemes might have additional effects (i.e bottlenecks), but these are not expected to significantly increase the rate of false positives in admixture tests [31]. The Tunisian population has been reported to be a genetic isolate [17] so it is plausible that part of the signal detected is actually due to genetic drift. However, this should not affect the other North African groups in our study. Finally, given that SNP arrays are based on common alleles and probably the relevant admixture information is encoded within the rare and very rare alleles, the potential bias, if anything, will underestimate ancient hominid admixture signals, as shown in previous studies [2],[3].

    This pattern was also observed by Meyer and colleagues earlier this year [2], and I discussed it in my post on that paper ("Denisova at high coverage"). Both papers note that ascertainment bias may contribute to this pattern. I added that Meyer and colleagues had assumed that genes found in sub-Saharan African populations could not have come from Neandertals, which greatly biased their estimates against Europe and West Asia, considering historical and prehistoric gene flow across the Sahara and along the Indian Ocean coast. So I'm not yet accepting the relative numbers of Neandertal ancestry from different populations, as we don't know that they have all come from consistent assumptions. In particular, an elevated amount of Neandertal ancestry in China -- this paper puts it almost as double the amount of Neandertal ancestry in northern Europeans -- is unlikely. There is no pattern of bottlenecks that can give rise to that excess without additional population mixture, and hard to see where such population mixture would have happened without also affecting the ancestors of Europeans. Instead, we have some work to do in reducing the biases on these comparisons.


    References

    Synopsis: 
    A study of North African genetic variation shows that Neandertal genes were widespread in the area before the Neolithic.
  • North by Northeast

    Tue, 2011-01-25 01:37 -- John Hawks

    An essay by Michael Balter in Science[1] asks the question, "Was North Africa the launch pad for modern human migrations?".

    This question seems to have an obvious answer. If you're in Africa and thinking about going somewhere else, you're going to have to go through the North part to get anywhere. South Africa seems like a really bad place to look for a "launch pad" of human migrations.

    Lots of people have written about South Africa as a "cradle" for modern human behavior. The density of high-quality archaeological sites explains this focus. The Howieson's Poort and Still Bay industries are genuinely interesting, and we can further examine a broader sampling of Middle Stone Age discoveries such as early adhesive use, heat-treated pressure flaking or cereal gathering. Still, most of these developments are very late in the game. If we're looking at things in South Africa 70,000 years ago, that's substantially later than the key events leading to human diversification within Africa.

    Balter reports on work that has during the past few years uncovered old dates for Aterian sites in North Africa. This regional variant of the Middle Stone Age is recognizable for its distinctive "tanged" points, and now extends from as early as 140,000 years ago to less than 40,000 years ago. The early end of this range is old enough to contribute to a possible dispersal of North/Northeast Africans into Eurasia. Hence Balter's story. On the other hand, if the Aterian were actually relevant to the movement of people into Eurasia, it is curious that Levantine Middle Paleolithic doesn't show clearer similarities to it.

    Most interesting detail: new skeletal material from Morocco:

    Last year, archaeologists excavating at the Grotte des Contrebandiers (Smuggler's Cave) on Morocco's Atlantic coast unearthed a rare prize: the skull and partial skeleton of a 7- or 8-year-old child. The fossils, dated to 108,000 years ago, appear to belong to an early member of our species, although study of them has just begun.

    I think it is increasingly likely that we will have genetics out of these North African materials. The fluctuating humidity of the Sahara (a focus of the article) does complicate matters, but the technology has progressed so rapidly that a well-preserved skeleton will surely turn up some endogenous DNA.

    Balter continues his story with some morphological analysis of North African materials. These don't as a group share any special similarities with Neandertals or people outside Africa -- although individual specimens do have features that show up elsewhere. The North African specimens (spanning from Jebel Irhoud at 160,000 years to Nazlet Khater at 40,000) are as you might expect really variable. They don't look particularly like recent peoples of North Africa, either -- "modern" in this context often means "not Neandertal" and masks some of the change that has taken place in the last 100,000 years. In other words, it's tough to tell a simple story of a North African ancestral population giving rise to variation outside of Africa, at least not without substantial evolutionary change in the non-Africans. It's not obvious how much of this evolution might be explained by mixture with Neandertals and other archaic Eurasians.

    Anyway, this explains why many paleoanthropologists don't see the Upper Paleolithic and equivalent-aged specimens outside of Africa as particularly African-looking. What remains unexplained is how much does morphology reflect ancestry over this kind of time span?

    I don't intend to answer the question, it needs more serious treatment.

    The genetic results have changed quickly over the past year. We will need to apply a somewhat different frame to North Africa -- one that recognizes a deeper differentiation of human populations within Africa (pre-Aterian, certainly, even with the older dates). We also have to resolve the biogeographic relationship across the Sinai between the Nile corridor and Levant.

    All of this means that Balter's story is very timely. Discovering more about the archaeology of Northeast Africa, the Arabian Peninsula and surrounding regions will obviously be crucial to understanding the rise of humans during the last 100,000 years.


    References

Subscribe to North Africa

Neandertals

For years, I've worked on their bones. Now I'm working on their genes. Read more about the science studying these ancient people.

Denisova

From a finger bone of an ancient human came the record of a completely unexpected population. My lab is working on the science of the Denisova genome.

Acceleration

The advent of agriculture caused natural selection to speed up greatly in humans. We're uncovering some of the ways that populations have rapidly changed during the last 10,000 years.

Malapa

Just outside Johannesburg, the Malapa site is producing some of the most exciting finds in human evolution. This site is the headquarters of the Malapa Soft Tissue Project.